tests: cover audio buffer decoding

[jamesachurchill]

Adds a small regression test for the in-memory audio decoding path used by server multipart uploads without ffmpeg conversion.

The test reads samples/jfk.wav into a byte buffer, decodes it through read_audio_data(const char *, size_t, ...), decodes the same sample through the existing filename path, and asserts the PCM outputs match.

This covers the core regression behind #3819 without requiring a server lifecycle, network harness, ffmpeg, or model download.

Local verification on aarch64:

• cmake --build build-3819-test --target test-read-audio-data-buffer -j2
• ctest --test-dir build-3819-test -R test-read-audio-data-buffer --output-on-failure

Result: 100% tests passed.

[danbev]

Should this be run in CI? (it would need to have the gh label if so):

Suggested change

	set_tests_properties(${AUDIO_BUFFER_TEST} PROPERTIES LABELS "unit")
	set_tests_properties(${AUDIO_BUFFER_TEST} PROPERTIES LABELS "unit;gh")

With that change it should have both labels:

$ cmake --workflow --preset cpu-debug
$ ctest --test-dir build-cpu-debug/ -R test-read-audio-data-buffer --output-on-failure --print-labels
Internal ctest changing into directory: /home/danbev/work/ai/whisper-work/build-cpu-debug
Test project /home/danbev/work/ai/whisper-work/build-cpu-debug
All Labels:
  gh
  unit